I'm puzzled that the recipe mentions peeling off the zest, then slicing lemons and combining with syrup & flowers. Apparently the zest is not used? The whole lemon, zest and all, is sliced and used in the recipe I use. I also use citric acid. Fruit Fresh, to be dusted over cut fruit to prevent browning, is the most readily available. And when I make mine it has a 3-day steeping period.

Peel the lemons, aiming to get only the yellow zest and not the white pith, and place in a heatproof boil along with the elderflowers. Squeeze the juice from the lemons and add to the bowl. Heat the sugar and water together, dissolving, and bring to the boil. Pour over the elderflowers and add the citric acid.. Leave overnight. Next day strain through a sieve and bottle. I tend to keep it in the fridge, or freeze it to keep even longer.

Never heard of steeping them in the syrup, that's not right. Here you go, my thirty year old method, never failed me.

1 colanderful of flowers, as much green removed as possible, but I don't worry too much

water to cover

sugar

Place flowers in pan and cover withcold water; simmer for about 15 minutes or so, strain, and measure the liquid. Return it to the pan and add 12 ox white sugar to each pint of liquid. Heat getnly until sugar is thoroughly dissolved. Leave to cool, then store/process/drink as desired.
Use diluted as squash; also makes lovely summery ice lollies

I made elderflower and rose..it needed steeping to get the most out of the rose petals..next time I'll use darker petals and steep longer to get colour and fragrance from the rose petal addition to the elderflowers